ارزیابی ساختار مکانی بارش در ارتباط با خشکسالی‌های ماهانۀ ایران

The present study identifies the monthly precipitation gravity center and investigates the routing patterns of these centers in Iran and its relationship with drought events. In this study, ERA5 monthly precipitation data over a period of 84 years (1940 to 2023) were used. First, spatial statistics of the dispersion and directional distribution of the precipitation gravity center were calculated, and then the movement path of the precipitation gravity center throughout the year was examined. Analysis of spatial changes in the precipitation gravity center in three time periods (the first period: 1940 to 1981, the second period: 1982 to 2023, and the third period: 1940 to 2023) was performed using a regression equation.

Finally, the latitude and longitude location of the precipitation gravity center was compared with the percentage of drought extent using the RAI index. The findings show that the axis of precipitation distribution in Iran tends from northwest to southeast in all months of the year. The precipitation gravity center is located in the path of the major precipitation systems in Iran, and its movement in different time periods has had significant effects on the precipitation pattern in different months in Iran. The movement of the precipitation gravity center in the second period (1982 to 2023) from December to March to the northwest led to a significant decrease in precipitation and an increase in the percentage of drought in the eastern half of Iran. Also, the movement of the precipitation gravity center in the months of November, April, and May to the southeast during the same period led to an increase in precipitation and a decrease in drought. The results show that an increase in the latitude of the precipitation gravity center is associated with an increase in the percentage of drought extent, while a decrease in latitude is associated with a decrease in drought.

1. Introduction

In this study, an attempt has been made to increase the accuracy of the spatial structure analysis of precipitation in Iran and to simulate the spatial distribution of precipitation comprehensively by using spatial statistical methods.

In this regard, the spatial statistical characteristics of precipitation, especially its concentration and dispersion components in different parts of the country, have been investigated.

Also, the path of precipitation gravity center changes has been examined and the trend of its changes in each month of the year in two 42-year periods (and its aggregation in an 84-year period) has been analyzed.

The main goal of these analyses has been to investigate the relationship between spatial changes in precipitation gravity center and the percentage of drought extent in Iran.

Considering the challenges arising from the vastness and geographical diversity of the country, this research is an effective step in improving the accuracy and validity of spatial precipitation analyses and can serve as a basis for developing analytical methods in future research.

Accordingly, the research questions are as follows: 1. Does the trajectory of precipitation intensity changes throughout the year in time intervals show specific patterns of spatial concentration and dispersion of precipitation in Iran? 2. What is the relationship between spatial changes in precipitation intensity and the percentage of drought in Iran?

In this study, ERA5 precipitation grid data from the ECMWF database were used. These data have a spatial resolution of 0.25 degrees of arc and a monthly temporal resolution for the period 1940 to 2023.

To implement the research process, three steps were carried out based on the above-mentioned database, which are referred to below:

First, the concept of precipitation centroid was used to determine the spatial distribution of the precipitation gravity center of concentration. The directional distribution of the precipitation gravity center of concentration for each month was extracted using GIS software. The directional distribution tool shows whether the distribution of geographical features in space is directional or not. The RAI index was used to assess drought in different regions.

The analysis of the spatial distribution map of monthly precipitation in Iran over the 84-year period (1940 to 2023) (Figure 2) shows the geographical and seasonal patterns of precipitation in the country. In winter (January to March), the precipitation gravity center is mainly concentrated in the western regions of the country and the Zagros mountainous region, and the precipitation pattern spreads from west to east. With the arrival of spring (April to June), the precipitation gravity center moves to the north and center of Iran, and in June it moves further south. In summer (July to September), precipitation is less and more scattered, and the precipitation gravity center remains in the northwestern regions of the Caspian Sea and the southern half. This phenomenon may indicate the dominance of the high pressure in Iran and clearly shows its effects on the climate of Iran during this period. In autumn (October to December), the precipitation gravity center returns to the northwest of Iran again, and the precipitation pattern spreads from the northwest to the southeast.

The map of changes in the precipitation gravity center in Iran in the three periods 1940-1981, 1982-2023, and 1940-2023 shows significant shifts in the precipitation gravity center. The largest shift occurred in December in the period 1940-1981 by 1.99 degrees to the northwest of the average.

This study investigated the spatial changes in the precipitation gravity center in Iran during three time periods: 1940–1981, 1982–2023, and 1940–2023. The results show that in recent decades, the precipitation gravity center has shifted to the northwest in the rainy months, especially December (about two degrees decrease in longitude and one-degree increase in latitude) and January (one-degree decrease in longitude and one and a half degrees increase in latitude).

The increase in the extent of drought in January and March, which are usually considered the rainy months in Iran, could indicate a decrease in water resources and an increase in climate hazards in these periods. In contrast, the decrease in the extent of drought in November and April has provided opportunities for adjusting climate conditions and better utilization of water resources in these months.